CDT++ owns random state at the simulation boundary. The root cdt::Random engine records a 64-bit seed and derives named PCG streams for independent subsystems:

stream 0 generates the initial triangulation;
stream 1 selects and constructs Metropolis-Hastings transitions.

Pass --seed SEED to cdt or initialize to replay the random inputs to a run. Without that option, the command obtains operating-system entropy once and prints the effective seed. Checkpoint and final OFF filenames also include seed-SEED; simulation checkpoints include pass-PASS. The seed is metadata in the filename rather than extra OFF content so files remain readable by standard CGAL triangulation parsers.

./out/build/reference/src/cdt -s -n640 -t4 -a0.6 -k1.1 -l0.1 -p10 --seed 92

./out/build/reference/src/initialize -s -n640 -t4 --seed 92

Distributions are short-lived operations applied to a caller-owned engine. Algorithms do not acquire entropy per sample, and tests use fixed seeds. The repository-owned Semgrep rules prevent new direct std::random_device or PCG engine construction outside Random.hpp. The supported spherical CGAL point generator receives a single seed derived from its caller-owned initialization stream rather than using CGAL's hidden default generator.

The reproducibility guarantee is exact for PCG draws and generated initialization points. Given the same starting manifold, the complete Metropolis transition sequence and counters also replay exactly. A freshly constructed triangulation is not promised to have byte-identical topology across CGAL versions, platforms, or builds: points on the same spherical layer are cospherical, so CGAL may choose a different valid tetrahedralization when resolving geometric ties.

Parallel stream policy

cdt::Random is not internally synchronized. One mutable engine belongs to one sequential run or one thread. Before parallel stochastic work is enabled, each worker must receive root.split(worker_stream) with a unique, stable stream identifier; engines must not be shared concurrently. PCG's stream selector gives independently parameterized sequences while retaining the root seed needed for replay. See the repository's PCG referencePCG reference.

Move-selection performance check

Issue #105 removes the old entropy-per-draw behavior from move-heavy paths. The benchmark retains that behavior only as a labeled baseline and compares it with one run-owned PCG stream:

just benchmark-rng 10000

The diagnostic reports both durations and their ratio. It is intentionally not a pass/fail CI test because operating-system entropy latency and runner load are machine-dependent; replay and distribution boundaries remain correctness tests in just ci.